1. Field of the Invention
This invention relates to a method for detecting a fibrous web tear in a drying section of a machine for producing the fibrous web, whereby the fibrous web is passed through the drying section by way of at least one dryer fabric, whereby the tear is detected by at least one optical web tear detection device which has a luminous source and a detector, and whereby a cut-off apparatus for the fibrous web is directly or indirectly activated by the web tear detection device.
In addition the invention relates to an apparatus for detecting a fibrous web tear in a drying section of a machine for producing the fibrous web, which is passed through the drying section by way of at least one dryer fabric, having at least one optical web tear detection device including a luminous source and a detector, and a cut-off apparatus for the fibrous web which is directly or indirectly activatable by the web tear detection device.
2. Description of the Related Art
During the production of all paper grades formed essentially from fibrous suspensions, the fast and reliable detection of tears in the fibrous web during its production is extremely important in order to prevent damage to parts of the machine used to produce the fibrous web.
At points at which the fibrous web is in free draw, web tears can be detected very reliably, for example by light barriers.
However, light barriers are unsuitable in situations in which the fibrous web rests on a skin acting as a web carrier, for example on a forming mesh, a press felt or a dryer fabric. Here use is made usually of optical systems on which a detector and a light or radiation source of the web tear detection device are mounted on the same side of the fibrous web.
Different web tear detection methods are known in this case.
For example, with the known “color detection” method, the difference in color between the fibrous web and the dryer fabric is used to detect the web tear. By way of example, FIG. 1 shows the signal flow, plotted in a color intensity/time diagram (F-t diagram), of an optical web tear detection device upon occurrence of a web tear. Upon overshooting or undershooting of a preselected trigger threshold value S (dashed line) of the color intensity F, a signal is sent to a cut-off apparatus. The trigger threshold value S for the signal to the cut-off apparatus can lie at for example 50% of the color intensity F. This method works particularly well with a distinct color difference such as that between a white fibrous web and a green dryer fabric for example.
A drawback of the color detection system is that it becomes more and more unreliable as the color difference between the fibrous web and the dryer fabric decreases. This is the case for example on machines for producing paperboard or packing paper, where brownish paper or paperboard webs lie on red or amber dryer fabrics. Here it is no longer possible to differentiate clearly between the fibrous web and the dryer fabric. This results in either web tear detection without a tear or in a tear without web tear detection. The former leads to an unnecessary stoppage of production and hence to financial loss for the plant operator, the latter runs the risk of causing damage to the machine.
Furthermore, dryer fabrics can become severely soiled the longer they are used, as the result of which the actual color of the dryer fabric is covered by dirt deposits. In such cases also it has been discovered that a color detection system no longer works reliably.
Another known method is the “detection of changes to light scatter (pseudo-structure detection)”, whereby light is scattered differently on the structured dryer fabric than on the fibrous web. This difference is used to detect web tears. The measurement takes place at wave lengths a little above the visible range (800 to 1,000 nm).
It has been discovered that pseudo-structure detection systems likewise do not work reliably. This is owed presumably to soiling in the dryer fabric or to the transparency of the still partly wet fibrous web.
Furthermore, the two methods mentioned are designed for narrow-band detection, meaning that measurements are taken within a wave length range of only a few nanometers, from approximately 20 to 100. Some sensor models permit the intensities of different wave length ranges to be combined to form one summation signal, such as happens for example in the addition of the three intensity channels of an RGB sensor. Here too the spectrum is not continuously detected however.
Narrow-band detection often results in only a small signal difference between the reflected intensities of the dryer fabric and the fibrous web being obtained. As previously explained, this is the case for example on machines for producing paperboard or packing paper, where brownish paper or paperboard webs lie on red or amber dryer fabrics. This can result in either web tear detection without a tear or in a tear without web tear detection. The former leads to an unnecessary stoppage of production and hence to financial loss for the plant operator, the latter runs the risk of causing damage to the machine.
Another disadvantage of narrow-band detection is that both the luminous source and the detector of the web tear detection device have to be moved very close to the measuring point, for example to within 10 to 15 cm, in order to obtain a reasonable signal level. As the result, the device could be damaged by the fibrous web disengaging from the web guide in the event of a tear.
In addition, the detector is subject over the long term to various influences: the color tone of the dryer fabric changes, dirt deposits form on the dryer fabric, and the moisture content of the fibrous web on the measuring point is not always constant. This can be held in check to a certain degree through regular readjustment of the detector. For this purpose the reference signals and the trigger threshold value can be reset, and the detection wave length range can also be adapted.
From the German publication DE 42 16 653 A1 there is known for example a method for detecting web tears, with which the change of color tone of the dryer fabric is corrected by automatic re-referencing.
What is needed in the art is a method and an apparatus of the types initially referred to such that a more reliable detection of web tears compared to the prior art is made possible in particular in single-row drying sections of a machine for producing a fibrous web.